The present invention relates to a printer capable of recording multiple gradation, and in more particularly, to a driving apparatus for a printer which is capable of recording multiple gradation and is usable as a hard copy apparatus for recording gradation image data coming from a computer and others, and to that printer.
Heretofore, in the case of a printer in which binary-coded gradation data are used for recording, on-off recording has been conducted, in accordance with binary-coded data, by a plurality of recording elements provided on a recording head. For example, in the case of an ink jet printer, a print head having thereon plural exhaust nozzles has been transmitting image data equivalent in terms of volume to a single jet of each nozzle to driving IC of the head each time, and the image data thus transmitted have been conducting ink jet for forming an image.
Recently, various printers have been conducting gradation recording, and an ink jet printer has also been conducting the gradation recording as in other printers. As an example of the gradation recording, there has been known an ink jet printer employing an error diffusion method. In the case of this ink jet printer, when outputting density of halftones, density of an ink drop constituting a dot is fixed, and the number of ink drops to be jetted to an area corresponding to plural pixels is changed, whereby the number of dots on an area spreading to some extent is changed to be approximated to the halftones.
On the other hand, a printer employing no error diffusion method has also been known. This method provides an effect similar to that brought about by changing dot sizes. In the case of this gradation recording, ink jet has been conducted for image forming through the same operations as those in the recording with conventional binary-coded gradation data wherein data of a pixel are constituted with plural bits, and signals equivalent to a single ink jet, namely, to one bit are subjected to serial transmission to a head by plural nozzles.
For example, Japanese TOKKOHEI 4-31220 discloses a technology wherein gradation data of plural bits are subjected to serial transfer to registers and latches, after they are converted to one bit, namely, converted to data corresponding in terms of quantity to one recording. Further, TOKKOHEI 4-74906 discloses an apparatus wherein gradation data of plural bits are converted to serial data and pulses having different number of pulses are generated from data equivalent to one pixel transmitted serially.
In technologies of TOKKOHEI 4-31220 and TOKKCOHEI 4-74906, however, image data are transmitted to a head on a serial basis, namely, on a basis of a unit of one bit, and recording requires longer time accordingly than in the occasion for recording binary-coded image data. For example, in the case of gradation recording for 16-gradation, the time required for transmission of signals to a head is 16 times that for the occasion which is not for gradation recording. Recording of gradation data is executed by repeating, for one pixel, the recording operations identical to those in binary-coded recording. When transmitting gradation data to a head serially, signals only for a single driving among recording operations for plural times are all that can be transmitted to the head, per one clock, for example.
Accordingly, when transmitting gradation data serially to a head in gradation recording, at least clocks in quantity of the gradations are required to complete the recording for one pixel, big problems such as signal transmission and a fall of recording speed caused by the signal transmission are caused, when compared with binary-coded recording wherein recording for one pixel can be completed with one clock.
Therefore, in the structure where image data are transmitted serially to a head in a multi-gradation printer, it is also possible to consider, for overcoming a fall of recording speed, the structure to raise transfer frequency and thereby to speed up recording operations of a head.
However, in an ink jet printer of the type wherein a head is installed on a carriage and scanning is made in the primary scanning direction for recording, another problem is caused. In the case of an ink jet printer or the like, a carriage moves together with a head and a driver for the head both installed on the carriage. A length of movement of the carriage depends on dimensions of a recording material, and it exceeds 20 cm, for example, in the ink jet printer which records on a recording sheet in A4 size. Therefore, the carriage and a control board are connected with a long and flexible cable. From a viewpoint of radio interference noise, it is not appropriate to transfer the high frequency data representing gradation data by connecting a carriage and a control board with a long and flexible cable as doing in an ink jet printer, because a noise is considered to be generated in the cable.
In addition, when providing a multi-gradation printer capable of outputting an image with high image quality, the number of gradations is increased causing an increase of the number of data, and driving frequency for a head is increased more and more, making it difficult to control.
As stated above, it is difficult to speed up a multi-gradation printer. In addition, even when speedup of a multi-gradation printer is realized, following problems are still foreseen.
In the case of an ink jet printer wherein gradation of an image to be outputted is controlled by the number of ink ejection, ejected several ink drops in quantity equivalent to one pixel are ejected continuously. Therefore, the several ink drops are stuck to be superposed at about the same position on a sheet. An extent of superposition on the superposed portion varies depending on the speed of relative movement between a recording medium such as a recording sheet and a nozzle and on the number of ejection of ink drops per a period of time.
For increasing the number of gradations, the number of ink drops to be ejected per one pixel is also increased. When the number of ink drops to be ejected per one pixel is increased, the superposed position where ink drops are overlapped each other is undesirably increased. In the case of an ink jet printer which records one pixel with ink drops in a single ejection like one employing an error diffusion method, it is possible to realize easily that no superposed portion is generated. However, in the ink jet printer wherein gradation of an image to be outputted is controlled by the number of ejection of ink, even when the number of ink drops is increased in an attempt to increase the number of gradations, the superposed portion is finally increased undesirably, resulting in difficult obtaining of gradation changes.
This trend appears more remarkably in the case of a special sheet for exclusive use such as a glossy sheet for ink jet use prepared for application to ink jet. These special sheets are coated for the purpose to prevent that ink soaks into a fiber of the sheet to run, expanding a dot to lower image quality. However, when attempting to control gradation of an image by the number of ink ejection, when hygroscopic property of ink is high, adhering ink drops soak into the sheet and color a wide area. For this reason, even when a superposed portion of ink drops is increased, a ratio of an area of no superposition is high, which makes it possible to obtain a gradation change slightly. However, ink drops adhering to a special sheet do not soak into the sheet but form one drop due to their surface tension undesirably. Therefore, a change of in area corresponding to adhering ink drops is small, and it is very difficult to obtain a gradation change, which is a problem.